1. Field of the Invention
The present invention relates to a battery control apparatus for controlling the battery installed in a hybrid vehicle, and relates in particular to a battery control apparatus to control charging or discharging amount according to the temperature of the battery.
2. Description of the Related Art
Conventionally, hybrid vehicles that carry a separate motor in addition to a drive engine as a power source are known. Hybrid vehicles are divided into a series-hybrid type and a parallel-hybrid type. The series-hybrid type vehicles in which the engine drives a generator that outputs electric power to operate the motor to drive the wheels.
Therefore, because the engine and the wheels are not linked, the engine can be operated at roughly a constant speed in a range of rotational speed to produce high fuel cost but low emission, so that better fuel cost and lower emission can be achieved compared with the conventional engine-driven vehicles.
In contrast, the parallel-hybrid vehicles in which a motor linked mechanically to the drive engine is used to assist the rotation of the drive shaft of the engine as well as to charge a storage battery using the same motor as a generator to store electrical charges in the battery, and the generated electricity is used also for electrical devices in the vehicle.
Therefore, in spite of the fact that the engine is linked to the wheels, the load on the engine can be reduced so that this type of vehicles can also achieve better low fuel cost and lower emission.
In the parallel-hybrid vehicles, there is a type in which the motor is connected directly to the output shaft of the engine, and the motor functions as the generator to charge the battery during deceleration of the vehicle, and another type in which either or both the engine and the motor are able to generate drive power, and a generator is provided separately.
In such hybrid vehicles, the demand of the driver can be met by having the motor assisting the output of the engine during the acceleration stage, or by having the motor charging the battery during the deceleration stage of the vehicle, due to "deceleration regeneration", for example, so that the electrical energy of the battery (referred to as the residual battery charge, hereinafter) can be maintained.
However, the battery used for hybrid vehicles exhibit a property that charging efficiency is rapidly lowered beyond a certain temperature. This temperature is dependent on the characteristics of the battery. If charging is continued when the battery is at a temperature higher than such a temperature, the charging energy is converted to heat and cannot be stored in the battery as electrical energy.
Further, if the battery is comprised by a number of modules, the capacity of each module can vary if the temperatures of the modules are not uniform. Variations in the module temperature can occur particularly easily when the battery temperature increases due to the effects of surrounding temperatures. Furthermore, when the battery temperature is high, it becomes difficult to detect the high-limit of residual battery charge, because of decrease in changes in the battery voltage required to detect the residual charge upper limit. For this reason, batteries in hybrid vehicles are often provided with a cooling facility for cooling the batteries so as to maintain the battery temperature below a transition temperature that charging efficiency is rapidly lowered.
However, the cooling facility can only function during the vehicle is motion, and therefore, when the vehicle is stationary, battery alone cannot be cooled. For this reason, the battery installed in the interior of the vehicle under the burning sun raises its temperature as the surrounding temperature rises. Under such a condition, even when the battery is cooled by moving the vehicle, it takes time to cool the battery temperature to less than the transition temperature. So long as the battery temperature remains high, charging does not produce storage of electrical energy in the battery so that it leads to a problem that the battery temperature can further increase.
Also, if charging or discharging is performed regarding that the temperature of the modules comprising the battery are uniform, though there are variations in the temperature of the modules, differences in the battery capacity can increase even more. Also, if charging is carried out while the battery is still at some high temperature, because the energy is transformed to heat and the battery voltage would not rise, it is difficult to detect the high-limit of residual battery charge from the change of the battery voltage, which leads to a problem of over-charging and degradation of the battery.
It is, therefore, an object of the present invention to provide an apparatus to control charge/discharge process of the battery dependent upon the temperature of the battery so as to prevent degradation of the battery.